Solid-state lighting devices such as light-emitting diodes (LEDs) are increasingly used in both consumer and commercial applications. Advancements in LED technology have resulted in highly efficient and mechanically robust light sources with long service life. Accordingly, modern LEDs have enabled a variety of new display applications and are being increasingly utilized for general illumination applications, often replacing incandescent and fluorescent light sources.
LEDs are solid-state devices that convert electrical energy to light, and they generally include one or more active layers of semiconductor material (forming at least one active region) arranged between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the one or more active layers where they recombine to generate emissions such as visible light or ultraviolet emissions. An active region may be fabricated, for example, from silicon carbide, gallium nitride, gallium phosphide, aluminum nitride, and/or gallium arsenide based materials and/or from organic semiconductor materials. Photons generated by the active region are omnidirectional in character.
Typically, it is desirable to operate LEDs at the highest light emission efficiency possible, which can be measured by the emission intensity in relation to the output power (e.g., in lumens per watt) in a desired direction of light. Therefore, a practical goal to enhance emission efficiency is to redirect the omnidirectional light emitted by the active region toward the desired direction. One way to increase light extraction efficiency in a desired direction is to provide reflective surfaces that reflect generated light so that such light may contribute to useful emission from an LED chip. In certain instances, the reflective surface is internal to the LED chip and the LED chip is mounted on a submount such that the reflective surface is between the active region of the LED and the submount. Accordingly, light emitted from the active region or internally reflected toward the submount is reflected back toward a primary light exiting surface. However, some light may be absorbed due to reflectivity values of less than 100% for various reflective surfaces. Some metals can have less than 95% reflectivity in wavelength ranges of interest. Additional LED chips have been developed with internal mirrors or reflectors that include structures permitting electrical signals to be passed through such mirrors or reflectors. Such structures can include various combinations of conductive features (e.g., layers and/or vias) and insulating features (e.g., dielectric and/or passivation layers).
In certain instances, an LED chip may be mounted on a reflective submount to further redirect light toward a desired light direction. In other instances, the LED chip and submount may be arranged inside a fixture that includes additional reflective surfaces.
The art continues to seek improved light-emitting diodes and solid-state lighting devices having reduced optical losses and providing desirable illumination characteristics capable of overcoming challenges associated with conventional lighting devices.